Radio receivers for receiving HDTV signals are described by the inventors in the U.S. patent applications listed below, incorporated by reference herein, and commonly assigned herewith:
Ser. No. 08/237,896 filed 4 May 1994 and entitled DIGITAL VSB DETECTOR WITH BANDPASS PHASE TRACKER, AS FOR INCLUSION IN AN HDTV RECEIVER issued 26 Dec. 1995 as U.S. Pat. No. 5,479,449;
Ser. No. 08/243,480 filed 19 May 1994 and entitled DIGITAL VSB DETECTOR WITH BANDPASS PHASE TRACKER USING RADER FILTERS, AS FOR USE IN AN HDTV RECEIVER;
Ser. No. 08/247,753 filed 23 May 1994 and entitled DIGITAL VSB DETECTOR WITH FINAL I-F CARRIER AT SUBMULTIPLE OF SYMBOL RATE, AS FOR HDTV RECEIVER;
Ser. No. 08/266,751 filed 28 Jun. 1994 and entitled HDTV RECEIVER WITH IMAGINARY-SAMPLE-PRESENCE DETECTOR FOR QAM/VSB MODE SELECTION, issued 9 Apr. 1996 as U.S. Pat. No. 5,506,636; and
Ser. No. 08/266,753 filed 28 Jun. 1994 and entitled RADIO RECEIVER FOR RECEIVING BOTH VSB AND QAM DIGITAL HDTV SIGNALS.
The invention described in these applications and in this disclosure are assigned to Samsung Electronics, Co., Ltd. pursuant to Employee Invention Agreements in effect as of the times these inventions were made.
In radio receivers for analog TV signals there is an interest in maintaining uniformity of amplitude response over a field and from field to field as well, since the amplitude response is directly used to control the intensity of light on a television viewing screen. In radio receivers for analog TV signals in which video carrier amplitude modulation decreases in energy with brighter signals, the horizontal sync pulses are the portions of the signal with largest excursion, so peak detection of these pulses is well suited to developing AGC voltages. Normally, the AGC in these receivers is quite "tight", which is to say that the level of sync tips in the video detector response is held reasonably constant over a wide range of signal strengths. When the video detector is an envelope detector there is a tendency for the AGC to "set up" on noise so that the level of sync tips in the video detector response is reduced with noise, which tendency to a degree may be desirable. When the video detector is a synchronous detector there is little if any tendency for the AGC to "set up" on noise.
In many radio receivers for amplitude-modulated or phase-modulated signals descriptive of digital symbols (such as QAM or PSK), "loose" AGC is developed simply using envelope detection of the IF signals. This "loose" AGC reduces intermediate-frequency signal levels responsive to received signals and their accompanying thermal background noise, so that clipping of modulation accompanied by such noise does not occur in the radio-frequency (RF) amplifier, intermediate-frequency (IF) amplifier, or detector portions of the radio receiver, or in the analog-to-digital converter(ADC). Strong impulse noise may be detected in an auxiliary receiver and used to gate a PIN switch that prevents the strong impulse noise from being applied as RF input signal to the principal radio receiver; this practice avoids ringing of the resonant circuits in the RF-amplifier and IF-amplifier portions of the radio receiver to curtail the lengths of time that digital data is disrupted by the strong impulse noise. In designs in which strong impulse noise is not gated out of RF input signal, the ringing resulting from the strong impulse noise may be clipped in an RF or IF amplifier, since data modulation is obliterated in any case. Preferably clipping is arranged to be symmetrical. In radio receivers for digital symbols there is less concern with maintaining uniformity of amplitude response over a field or over a sequence of fields than there is in an analog TV receiver, since the amplitude response is not directly used to control the intensity of light on a television viewing screen. Adaptive or "soft-decision" symbol decoding techniques which adjust symbol-level decoding ranges depending on received signals (plus noise) are customarily used when the symbol codes have multiple levels.
Alternatively, the symbol-level decoding ranges may have prescribed boundaries, and the baseband signal supplied from the final detector in the radio receiver may be adjusted in level using an automatically gain-controlled (AGCd) amplifier that has "tight" AGC. The AGCd amplifier can be a baseband amplifier in cascade connection after the final detector of the radio receiver, which final detector is an ADC in some designs. The baseband AGCd amplifier will also follow any ADC used to quantize the response of the final detector of the radio receiver if that final detector is of a type that supplies an analog response.
A background problem in digital HDTV receivers is being able to switch rapidly from one channel to another, so that a viewer can perform channel-to-channel selection without annoying delay when hunting for something he wishes to view. Symbol decoding is done using a trellis decoder and is preferably adaptive or "soft-decision" in nature. When a new channel is selected, there are delays associated with symbol synchronization, with adjusting the symbol-level ranges in the trellis decoder, and with the decoding taking place after trellis decoding, particularly the MPEG-2 decoding used to de-compress compressed digital data. The MPEG-2 decoders currently used have a one second or so delay associated with them, which is about all the delay that is tolerable during channel-to-channel selection. So it is very desirable to minimize the time required for adjusting the response of the final detector of the radio receiver and the extent of symbol-level ranges in the trellis decoder respective to each other. Prescribing the boundaries of the symbol-level decoding ranges and providing "tight" AGC of the IF-amplifier response supplied to the final detector of the radio receiver portion of the digital HDTV receiver is preferred by the inventors. This avoids any delay associated with adjusting the boundaries of the symbol-level decoding ranges because of having to wait for the radio receiver AGC system to settle. This also avoids any delay associated with the use of separate AGC systems for the radio receiver and for the symbol codes supplied from the final detector of that radio receiver. Tracking problems between two different systems for adjusting gain are also avoided.
The departures of the symbol code levels in the final detector response of the radio receiver, from closest midpoints in the symbol-level decoding ranges, can be detected in order to generate the AGC signal (or, alternatively, to adjust the boundaries of the symbol-level decoding ranges). There are especial characteristics of the data received by digital HDTV receivers that allow simpler development of AGC signals (or, alternatively, simpler adjustment of the boundaries of the symbol-level decoding ranges), however.
In the 6 MHz-bandwidth digital HDTV signals to be used in the United States, each data field contains 314 data lines, and the fields are consecutively numbered modulo-two in order of their occurrence. Each line of data starts with a line synchronization code group of four symbols having successive values of +S, -S, -S and +S. The value +S is one level below the maximum positive data excursion, and the value -S is one level above the maximum negative data excursion. The lines of data are each of 77.7 microsecond duration, and there are 832 symbols per data line for a symbol rate of about 10 megabits/second. The initial line of each data field is a field synchronization code group that codes a training signal for channel-equalization and multipath suppression procedures. The training signal is a 511-sample pseudo-random sequence (or "PR-sequence") followed by three 63-sample PR sequences. This training signal is transmitted in accordance with a first logic convention in the first line of each odd-numbered data field and in accordance with a second logic convention in the first line of each even-numbered data field, the first and second logic conventions being one's complementary respective to each other.
Developing AGC in an HDTV receiver proceeding from detection of line synchronization code group suggests itself by analogy to developing AGC in an analog TV receiver proceeding from detection of horizontal line synchronization pulses. In HDTV signals the data line synchronization code groups are not the portions of the signal with largest excursion, however, and code groups resembling line synchronization code groups can occur at times within data lines. So, for AGC purposes, detecting the amplitude of the data line sync pulse groups is not entirely analogous to detecting the amplitude of horizontal sync pulses in an analog TV signal. U.S. Pat. No. 5,410,368 issued 25 Apr. 1995 to G. Krishnamurthy et alii, entitled CARRIER ACQUISITION BY APPLYING SUBSTITUTE PILOT TO A SYNCHRONOUS DEMODULATOR DURING A START UP INTERVAL and assigned to Zenith Electronics Corp. describes the generation of AGC signals from match filter response to the data line sync pulse groups. The match filter exhibits peaks in its response when data line sync pulse groups occur, so AGC signals can be developed from these response peaks in a way analogous to the way AGC signals are developed in an analog TV receiver proceeding from horizontal line synchronization pulses.
Vestigial sideband (VSB) signals that will be used in terrestrial broadcast transmissions of HDTV signal in the United States employ twelve interleaved trellis codes, each a 2/3rate trellis code with one uncoded bit, which interleaved trellis codes are transmitted as 8-level (3 bit) one-dimensional-constellation symbol coding. The VSB signals have their natural carrier wave, which would vary in amplitude depending on the percentage of modulation, suppressed. The natural carrier wave is replaced by a pilot carrier wave of fixed amplitude, which amplitude corresponds to a prescribed percentage of modulation. This pilot carrier wave of fixed amplitude is generated by introducing a direct (or "zero-frequency") component of shift into the modulating voltage applied to the balanced modulator generating the amplitude-modulation sidebands that are supplied to the filter supplying the VSB signal as its response. If the eight levels of the symbol coding have normalized values of -7, -5, -3, -1, +1, +3, +5 and +7 in the carrier modulating signal, the pilot carrier has a normalized vale of 1.25. The normalized value of +S is +5, and the normalized value of -S is -5.
Since the pilot carrier wave is continuously transmitted together with VSB HDTV signals and has an amplitude that is a prescribed percentage of modulation, very good AGC signals can be developed by synchronously detecting the pilot carrier wave, the inventors point out. The synchronous detector response can be lowpass filtered to strongly select against noise accompanying the direct, or zero-frequency, component of synchronous detector response used for developing AGC signal. Strong selection against noise is considerably simpler to achieve than in AGC systems that base AGC on detecting envelope variations. The strong selection against noise that is possible makes "very tight" AGC possible. Since the amplitude of the pilot carrier wave is in direct proportion to changes in symbol code level, this "very tight" AGC can accurately control the symbol levels supplied to the symbol decoder so they are centered within prescribed symbol ranges.
Since the pilot carrier wave is continuously transmitted; and since data modulation has no direct, or zero-frequency, component; AGC time constants can be made substantially shorter than a data line interval when AGC signal is generated by narrowband synchronous detection of the pilot carrier. AGC is very quickly established.